1. Field of the Invention
The present invention relates to a photocatalyst containing metallic ultrafine particles, a supported catalyst in which the above-mentioned photocatalyst containing metallic ultrafine particles is supported on a carrier and processes for producing the photocatalyst and the supported catalyst. In more detail the present invention is concerned with a photocatalyst containing metallic ultrafine particles which comprises a substrate such as fine particles having a photocatalytic function and metallic nanocolloid particles that are supported on the substrate by the use of a metallic nanocolloid liquid substantially free from a protective colloid formation agent; a supported catalyst in which the photocatalyst containing metallic ultrafine particles is supported on a carrier; and processes for producing the photocatalyst and the supported catalyst.
2. Description of the Related Arts
In recent years, attention has been focused upon the methods which carry out deodorization, antimicrobials, disinfection, antifouling, removal of harmful material, prevention against dimming of glass and mirror and the like by utilizing the activities of a photocatalyst.
A photocatalytically activated material (hereinafter sometimes referred simply to as “photocatalyst”), when being irradiated with rays having energy not lower than its band gap, is excited to generate electrons on conductive bands and also positive holes on valence electron bands. It is known that the electrons thus generated reduce surface oxygen to form superoxide anions (.O2−), while the positive holes oxidize surface hydroxyl groups to form hydroxyl radicals (.OH), and these reactively activated oxygen species exhibit a strong oxidizing decomposition function, thereby decomposing in high efficiency, organic substances that are attached to the surfaces of the photocatalyst.
Practically useful photocatalytically activated material is titanium dioxide, particularly anatase-type titanium dioxide. Further it is a general practice to install on a layer of the photocatalytically activated material, a shielding layer of a platinum series metal such as platinum, palladium, rhodium, ruthenium or the like for the purpose of accelerating the activities of the photocatalyst. The platinum series metal to be added thereto functions as a promoter for the photocatalyst, and plays a role in enhancing the oxidizing activity therefore. That is to say, the titanium oxide based photocatalyst which is incorporated with a noble metal such as platinum has strong photooxidizing power, thereby completely oxidizes almost all organic substances, and exhibits such effect as exterminating microorganisms by photodecomposing the surfaces thereof. Thus various proposals have been made on air cleaning filters that utilize the above-mentioned composite catalyst in which a platinum series catalyst is supported on the photocatalyst (Japanese Patent Application Laid-Open No. 71323/1998 (Heisei 10), Claims).
Various proposals have been made on the method for supporting metallic particles on the photocatalyst, and are exemplified by immersion method and photoelectrodeposition method. The immersion method is a method which comprises immersing photocatalyst particles of titanium oxide or the like in an aqueous solution of a metal with a proper concentration during a prescribed period of time, so that the metallic salt is supported on the photocatalyst and thereafter, reducing the metallic salt with a reducing agent (for instance, refer to Japanese Patent Application Laid-Open No. 71323/1998 (Heisei 10), paragraph 0025).
However, the immersion method usually necessitates heating upon reducing a metal, and the heating sometimes causes aggregation of supported metallic fine particles. Such aggregation thereof, when deteriorates the dispersion state of the metallic particles, lowers photocatalytic activity. In addition from the viewpoint of production of the photocatalyst, the method involves the problem in that there is required a unit for heating at the time of reduction, which becomes a factor of an increase in initial cost, while environmental consideration is made necessary depending upon volatile components generated by heating, whereby a supporting method and environment upon the supporting are unfavorably limited.
The photoelectrodeposition method is a method which comprises dispersing photocatalyst particles of titanium oxide or the like in a solution containing a metal in a proper amount, and irradiating the same with ultraviolet ray for a proper period of time in the presence of a small amount of a reducing agent such as an alcohol or the like, so that the metal is supported on the photocatalyst by the photoreducing power of the photocatalyst powders (for instance, refer to Japanese Patent Application Laid-Open No. 71323/1998 (Heisei 10), paragraph 0026).
Nevertheless, it is difficult for the photoelectrodeposition method to impart uniformity in a nanolevel to the supported metallic particles, thereby bringing about difficulty in controlling the particle diameter and dispersibility of the particles.
Moreover there is disclosed, as an other method for supporting metallic particles on the photocatalyst, a method which comprises immersing the photocatalyst in a metallic nanocolloid liquid containing metallic nanocolloid fine particles, so that the metallic particles are supported on the photocatalyst. The above-mentioned method has advantages of simple operation, dispensing with an expensive coating unit and the like. By the term “nanocolloid” as used herein is meant colloid particles having an equivalent diameter of smaller than 100 nm, approximately.
Notwithstanding the advantage, since the above-mentioned metallic nanocolloid liquid is generally poor in dispersion stability of metallic nanocolloid particles, and is liable to cause aggregation, there are usually taken measures for enhancing dispersion stability thereof through the formation of a protective colloid by adding a protective colloid formation agent such as water soluble high molecular compound exemplified by polyvinyl alcohol, polyvinyl pyrrolidone or gelatin, or a surfactant.
For instance, there is disclosed a process for producing noble metal fine particles supported on photocatalyst thin films which comprises coating thin films having a photocatalytic function with noble metal fine particulate colloid which is stabilized with a surfactant and thereafter heat treating the coating at about 400 to 600° C. in a reducing atmosphere for instance, refer to Japanese Patent Application Laid-Open No. 71137/1999 (Heisei 11), Claims) and the like.
However, when use is made of the protective colloid formation agent in such a manner in the case of metallic nanocolloid particles being supported on a substrate, the nanocolloid particles are supported with the protective colloid formation agent attached as such to the surfaces thereof, whereby it follows that the resultant carrier of the metal contains organic substances as well. Such carrier of metal containing the organic substances sometimes fails to sufficiently exert the objective functions and in this case, necessitates measures for removing the organic substances by means of a firing treatment. Further a firing treatment is impossible depending upon the type of substrate, thereby bringing about the problem in that the type of substrate is inevitably restricted.
There is known, as a process for producing metallic nanocolloid liquid without the use of a protective colloid formation agent, for instance, a method which comprises adding a reducing agent in a solution in which a metal salt is dissolved, so that metallic fine particles are formed by reducing metallic ions (for instance, refer to Japanese Patent Application Laid-Open No. 224969/2001 (Heisei 13) and Surface, Vol. 21, No. 8 pp 450 to 456 (1983)).
In regard to a method for supporting metallic fine particles on a photocatalyst, in order to support the particles in a definite amount or more, metallic nanocolloid particles in an amount of some extent are preferably supported from the aspect of operation, whereby it is required to increase the colloid particles concentration to some extent in the metallic nanocolloid liquid.
Nevertheless with regard to a process for producing metallic nanocolloid liquid without the use of a protective colloid formation agent, when an attempt is made to prepare metallic nanocolloid liquid containing high concentration metallic nanocolloid particles, there is created a problem in that aggregative precipitation of colloid particles easily takes place. For instance, in the case where platinum is used as a metal without using a protective colloid formation agent, the limiting concentration of metallic nanocolloid solution capable of being prepared has been 150 ppm by mass, approximately.